The offer of criminal amnesty to 1,500 Long Island Rail Road retirees on disability pensions might mean all the major players have been ensnared in a major federal investigation, legal observers said yesterday.

But attorneys representing some of those charged said the offer mailed yesterday to pension abusers who have not yet been charged shows the weakness of the federal government’s case against the 11 people indicted last year.

“My guess is they’ve made their cases on the most serious offenders, and they’re just looking to mop up,” said Fred Klein, a professor at Hofstra Law School and former Nassau County prosecutor.